Table showing the cost of various foods. Using this table, make up an economical dietary for one day, three meals, for a man doing moderate work. Give reasons for the amount of food used and for your choice of foods. Make up another dietary in the same manner, using expensive foods. What is the difference in your bill for the day?
A Mixed Diet Best.—Knowing the proportion of the different food substances required by man, it will be an easy matter to determine from the tables and charts shown you the best foods for use in a mixed diet. Meats contain too much nitrogen in proportion to the other substances. In milk, the proportion of proteins, carbohydrates, and fats is nearly right to make protoplasm; a considerable amount of mineral matter being also present. For these reasons, milk is extensively used as a food for children, as it combines food material for the forming of protoplasm with mineral matter for the building of bone. Some vegetables (for example, peas and beans) contain a large amount of nitrogenous material but in a less digestible form than is found in some other foods. Vegetarians, then, are correct in theory when they state that a diet of vegetables may contain everything necessary to sustain life. But a mixed diet containing meat is healthier. A purely vegetable diet contains much waste material, such as the cellulose forming the walls of plant cells, which is indigestible. It has been recently discovered that the outer coats of some grains, as rice, contain certain substances (enzymes) which aid in digestion. In the case of polished rice, when this outer coat is removed the grain has much less food value.
Daily Fuel Needs of the Body.—It has been pointed out that the daily diet should differ widely according to age, occupation, time of year, etc. The following table shows the daily fuel needs for several ages and occupations:—
| Daily Calorie Needs (Approximately) | ||
|---|---|---|
| 1. | For child under 2 years | 900 Calories |
| 2. | For child from 2-5 years | 1200 Calories |
| 3. | For child from 6-9 years | 1500 Calories |
| 4. | For child from 10-12 years | 1800 Calories |
| 5. | For child from 12-14 (woman, light work, also) | 2100 Calories |
| 6. | For boy (12-14), girl (15-16), man, sedentary | 2400 Calories |
| 7. | For boy (15-16) (man, light muscular work) | 2700 Calories |
| 8. | For man, moderately active muscular work | 3000 Calories |
| 9. | For farmer (busy season) | 3200 to 4000 Calories |
| 10. | For ditchers, excavators, etc. | 4000 to 5000 Calories |
| 11. | For lumbermen, etc. | 5000 and more Calories |
Normal Heat Output.—The following table gives the result of some experiments made to determine the hourly and daily expenditure of energy of the average normal grown person when asleep and awake, at work or at rest:—
| Average Normal Output of Heat from the Body | |
|---|---|
| Conditions of Muscular Activity | Average Calories per Hour |
| Man at rest, sleeping | 65 Calories |
| Man at rest, awake, sitting up | 100 Calories |
| Man at light muscular exercise | 170 Calories |
| Man at moderately active muscular exercise | 290 Calories |
| Man at severe muscular exercise | 450 Calories |
| Man at very severe muscular exercise | 600 Calories |
It is very simple to use such a table in calculating the number of Calories which are spent in twenty-four hours under different bodily conditions. For example, suppose the case of a clerk or school teacher leading a relatively inactive life, who
| sleeps for 9 hours | × | 65 Calories = 585 |
| works at desk 9 hours | × | 100 Calories = 900 |
| reads, writes, or studies 4 hours | × | 100 Calories = 400 |
| walks or does light exercise 2 hours | × | 170 Calories = 340 |
| 2225 | ||
This comes out, as we see, very close to example 6 of the table[39] on page 284.
How we may Find whether we are Eating a properly Balanced Diet.—We already know approximately our daily Calorie needs and about the proportion of protein, fat, and carbohydrate needed. Dr. Irving Fisher of Yale University has worked out a very easy method of determining whether one is living on a proper diet. He has made up a number of tables, in which he has designated portions of food, each of which furnishes 100 Calories of energy. The tables show the proportion of protein, fat, and carbohydrate in each food, so that it is a simple matter by using such a table to estimate the proportions of the various nutrients in our dietary. We may depend upon taking somewhere near the proper amount of food if we take a diet based upon either Atwater's, Chittenden's, or Voit's standard. One of the most interesting and useful pieces of home work that you can do is to estimate your own personal dietary, using the tables giving the 100-Calorie portion to see if you have a properly balanced diet. From the table on page 286 make out a simple dietary for yourself for one day, estimating your own needs in Calories and then picking out 100-Calorie portions of food which will give you the proper proportions of protein, fat, and carbohydrate.
Transcriber's Note—Column header
abbreviations for table, below:
Wt. - Weight in Ounces, 100
Calorie Portion
Carbs. - Carbohydrates
Por. - 100 Calorie
Portion
| Table of 100 Calorie Portions—Modified from Fisher | |||||||
|---|---|---|---|---|---|---|---|
| Calories Furnished | Price | ||||||
| Food | Portion
Containing 100 Calories |
Wt. | Pro- tein |
Fat | Carbs. | 1 Lb. | Por. |
| Oysters | 1 doz. | 6.8.0 | 49 | 22.0 | 29 | .175 | .07 |
| Bean soup | ½ small serving | 2.60 | 24 | 12.0 | 64 | .007 | |
| Cream of corn | 2/3 ordin. serv. | 3.10 | 11 | 58.0 | 31 | .02 | |
| Vegetable soup | ½ ordin. serv. | 2.40 | 8 | 89.0 | 3 | .01 | |
| Cod fish (fresh) | ordin. serv. | 5.00 | 95 | 5.0 | 0 | .12 | .04 |
| Salmon (canned) | small serv. | 1.75 | 45 | 55.0 | 0 | .22 | .03 |
| Chicken | ½ large serv. | 1.75 | 39 | 56.0 | 5 | .22 | .05 |
| Veal cutlet | 2/3 large serv. | 2.40 | 54 | 46.0 | 0 | .28 | .045 |
| Beef, corned | ½ large serv. | 1.00 | 15 | 85.0 | 0 | .16 | .01 |
| Beef, sirloin | small serv. | 1.60 | 33 | 67.0 | 0 | .34 | .04 |
| Beef, round | small serv. | 1.80 | 39 | 61.0 | 0 | .24 | .025 |
| Ham, lean | ordin. serv. | 1.10 | 28 | 72.0 | 0 | .22 | .015 |
| Lamb chops | ½ ordin. serv. | 1.00 | 24 | 76.0 | 0 | .20 | .013 |
| Mutton, leg | ordin. serv. | 1.20 | 35 | 65.0 | 0 | .20 | .015 |
| Eggs, boiled | 1 large egg | 2.10 | 32 | 68.0 | 0 | .30 doz. | .025 |
| Eggs, scrambled | 1-1/3 ordin. serv. | 2.50 | 37 | 58.0 | 5 | .30 doz. | .03 |
| Beans, baked | side dish | 2.66 | 21 | 18.0 | 61 | .08 | .013 |
| Potatoes, mashed | ordin. serv. | 3.20 | 10 | 25.0 | 65 | .02 | .005 |
| Macaroni | 1/3 large serv. | .95 | 15 | 3.0 | 82 | .10 | .01 |
| Potato salad | ordin. serv. | 2.25 | 10 | 57,0 | 33 | .20 | .025 |
| Tomatoes, sliced | 4 large serv. | 15.00 | 15 | 16.0 | 69 | .10 | .10 |
| Rolls, plain | 1 large roll | 1.20 | 12 | 7.0 | 81 | .10 doz. | .01 |
| Butter | ordin. pat | .44 | 5 | 99.5 | .35 | .01 | |
| Wheat bread | 1 small slice | .96 | 15 | 5.0 | 80 | .07 | .005 |
| Chocolate cake | ½ ord. sq. piece | .98 | 7 | 22.0 | 71 | .32 | .02 |
| Gingerbread | ½ ord. sq. piece | .96 | 6 | 23.0 | 71 | .16 | .01 |
| Custard pudding | ordin. serv. | 3.25 | 18 | 42.0 | 40 | .15 | .03 |
| Rice pudding | very small serv. | 2.65 | 8 | 13.0 | 79 | .13 | .02 |
| Apple pie | 1/3 piece | 1.30 | 5 | 32.0 | 63 | .013 | |
| Cheese, American | 1-½ cu. in. | .77 | 25 | 73.0 | 2 | .19 | .01 |
| Crackers (soda) | 2 crackers | .90 | 10 | 20.0 | 70 | .10 | .007 |
| Currant jelly | 2 heap. spoons | 1.10 | 2 | 0.0 | 98 | .40 | .025 |
| Sugar | 3 teaspoons | .86 | 0 | 0.0 | 100 | .06 | .003 |
| Milk as bought | small glass | 4.90 | 19 | 52.0 | 29 | .05 | .015 |
| Milk, cond., sweet | 4 teaspoons | 1.06 | 10 | 23.0 | 67 | .01 | |
| Oranges | 1 large one | 9.40 | 6 | 3.0 | 91 | .025 | |
| Peanuts | 13 double ones | .62 | 20 | 63.0 | 17 | .004 | |
| Almonds, shelled | 8-15 | .53 | 13 | 77.0 | 10 | .025 | |
From the preceding table plan a well-balanced and cheap dietary for one day for a family of five, two adults and three children. Make a second dietary for the same time and same number of people which shall give approximately the same amount of tissue and energy producing food from more expensive materials.
Food Waste in the Kitchen.—Much loss occurs in the improper cooking of foods. Meats especially, when overdone, lose much of their flavor and are far less easily digested than when they are cooked rare. The chief reasons for cooking meats are that the muscle fibers may be loosened and softened, and that the bacteria or other parasites in the meat may be killed by the heat. The common method of frying makes foods less digestible. Stewing is an economical as well as healthful method. A good way to prepare meat, either for stew or soup, is to place the meat, cut in small pieces, in cold water, and allow it to simmer for several hours. Rapid boiling toughens the muscle fibers by the too rapid coagulation of the albuminous matter in them, just as the white of egg becomes tough when boiled too long. Boiling and roasting are excellent methods of cooking meat. In order to prevent the loss of the nutrients in roasting, it is well to baste the meat frequently; thus a crust is formed on the outer surface of the meat, which prevents the escape of the juices from the inside.
Vegetables are cooked in order that the cells containing starch grains may be burst open, thus allowing the starch to be more easily attacked by the digestive fluids. Inasmuch as water may dissolve out nutrients from vegetable tissues, it is best to boil them rapidly in a small amount of water. This gives less time for the solvent action to take place. Vegetables should be cooked with the outer skin left on when it is possible.
Adulterations in Foods.—The addition of some cheaper substance to a food, or the subtraction of some valuable substance from a food, with the view to cheating the purchaser, is known as adulteration. Many foods which are artificially manufactured have been adulterated to such an extent as to be almost unfit for food, or even harmful. One of the commonest adulterations is the substitution of grape sugar (glucose) for cane sugar. Glucose, however, is not a harmful adulterant. It is used largely in candy making. Flour and other cereal foods are sometimes adulterated with some cheap substitutes, as bran or sawdust. Alum is sometimes added to make flour whiter. Probably the food which suffers most from adulteration is milk, as water can be added without the average person being the wiser. By means of an inexpensive instrument known as a lactometer, this cheat may easily be detected. In most cities, the milk supply is carefully safeguarded, because of the danger of spreading typhoid fever from impure milk (see Chapter XX). Before the pure food law was passed in 1906, milk was frequently adulterated with substances like formalin to make it keep sweet longer. Such preservatives are harmful, and it is now against the law to add anything whatever to milk.
Coffee, cocoa, and spices are subject to great adulteration; cottonseed oil is often substituted for olive oil; butter is too frequently artificial; while honey, sirups of various kinds, cider and vinegar, have all been found to be either artificially made from cheaper substitutes or to contain such substitutes.
Pure Food Laws.—Thanks to the National Pure Food and Drug Law passed by Congress in 1906, and to the activity of various city and state boards of health, the opportunity to pass adulterated foods on the public is greatly lessened. This law compels manufacturers of foods or medicines to state the composition of their products on the labels placed on the jars or bottles. So if a person reads the label he can determine exactly what he is getting for his money.
Impure Water.—Great danger comes from drinking impure water. This subject has already been discussed under Bacteria, where it was seen that the spread of typhoid fever in particular is due to a contaminated water supply. As citizens, we must aid all legislation that will safeguard the water used by our towns and cities. Boiling water for ten minutes or longer will render it safe from all organic impurities.
Stimulants.—We have learned that food is anything that supplies building material or releases energy in the body; but some materials used by man, presumably as food, do not come under this head. Such are tea and coffee. When taken in moderate quantities, they produce a temporary increase in the vital activities of the person taking them. This is said to be a stimulation; and material taken into the digestive tract, producing this, is called a stimulant. In moderation, tea and coffee appear to be harmless. Some people, however, cannot use either without ill effects, even in small quantity. It is the habit formed of relying upon the stimulus given by tea or coffee that makes them a danger to man. Cocoa and chocolate, although both contain a stimulant, are in addition good foods, having from 12 per cent to 21 per cent of protein, from 29 per cent to 48 per cent fat, and over 30 per cent carbohydrate in their composition.
Is Alcohol a Food?—The question of the use of alcohol has been of late years a matter of absorbing interest and importance among physiologists. A few years ago Dr. Atwater performed a series of very careful experiments by means of the respiration calorimeter, to ascertain whether alcohol is of use to the body as food.[40] In these experiments the subjects were given, instead of their daily allotment of carbohydrates and fats, enough alcohol to supply the same amount of energy that these foods would have given. The amount was calculated to be about two and one half ounces per day, about as much as would be contained in a bottle of light wine.[41] This alcohol was administered in small doses six times during the day. Professor Atwater's results may be summed up briefly as follows:—
1. The alcohol administered was almost all oxidized in the body.
2. The potential energy in the alcohol was transformed into heat or muscular work.
3. The body did about as well with the rations including alcohol as it did without it.
The committee of fifty eminent men appointed to report on the physiological aspects of the drink problem reported that a large number of scientific men state that they are in the habit of taking alcoholic liquor in small quantities, and many report that they do not feel harm thereby. A number of scientists seem to agree that within limits alcohol may be a kind of food, although a very poor food.
On the other hand, we know that although alcohol may technically be considered as a food, it is a very unsatisfactory food and, as the following statements show, it has an effect on the body tissues which foods do not have.
Professor Chittenden of Yale College, in discussing the food problem of alcohol, writes as follows:
"It is true that alcohol in moderate quantities may serve as a food, i.e. it can be oxidized with the liberation of heat. It may to some extent take the place of fat and carbohydrates, but it is not a perfect substitute for them, and for this reason alcohol has an action that cannot be ignored. It reduces liver oxidation. It therefore presents a dangerous side wholly wanting in carbohydrates and fat. The latter are simply burned up to carbonic acid and water or are transformed to glycogen and fat, but alcohol, although more easily oxidized, is at all times liable to obstruct, in a measure at least, the oxidative processes of the liver and probably of other tissues also, thereby throwing into the circulation bodies, such as uric acid, which are harmful to health, a fact which at once tends to draw a distinct line of demarcation between alcohol and the two non-nitrogenous foods, fat and carbohydrates. Another matter must be emphasized, and it is that the form in which alcohol is taken is of importance. Port wine, for instance, has more influence on the amount of uric acid secreted than an equivalent amount of alcohol has in some other form. To conclude: as an adjunct to the ordinary daily diet of the healthy man alcohol cannot be considered as playing the part of a true non-nitrogenous food."—Quoted in American Journal of Inebriety, Winter, 1906.
Effect of Alcohol on Living Matter.—If we examine raw white of egg, we find a protein which closely resembles protoplasm in its chemical composition; it is called albumen. Add to a little albumen in a test tube some 95 per cent alcohol and notice what happens. As soon as the alcohol touches the albumen the latter coagulates and becomes hard like boiled white of egg. Shake the alcohol with the albumen and the entire mass soon becomes a solid. This is because the alcohol draws the water out of the albumen. It has been shown that albumen is somewhat like protoplasm in structure and chemical composition. Strong alcohol acts in a similar manner on living matter when it is absorbed by the living body cells. It draws water from them and hardens them. It has a chemical and physical action upon living matter.
Alcohol a Poison.—But alcohol is also in certain quantities a poison. A commonly accepted definition of a poison is that it is any substance which, when taken into the body, tends to cause serious detriment to health, or the death of the organism. That alcohol may do this is well known by scientists.
It is a matter of common knowledge that alcohol taken in small quantities does not do any apparent harm. But if we examine the vital records of life insurance companies, we find a large number of deaths directly due to alcohol and a still greater number due in part to its use. In the United States every year there are a third more deaths from alcoholism and cirrhosis of the liver (a disease directly caused by alcohol) than there are from typhoid fever. The poisonous effect is not found in small doses, but it ultimately shows its harmful effect. Hardening of the arteries, an old-age disease, is rapidly becoming in this country a disease of the middle aged. From it there is no escape. It is chiefly caused by the cumulative effect of alcohol. The diagram following, compiled by two English life insurance companies that insure moderate drinkers and abstainers, shows the death rate to be considerably higher among those who use alcohol.
Abstainers live longer than moderate drinkers.
Dr. Kellogg, the founder of the famous Battle Creek Sanitarium, points out that strychnine, quinine, and many other drugs are oxidized in the body but surely cannot be called foods. The following reasons for not considering alcohol a food are taken from his writings:—
"1. A habitual user of alcohol has an intense craving for his accustomed dram. Without it he is entirely unfitted for business. One never experiences such an insane craving for bread, potatoes, or any other particular article of food.
"2. By continuous use the body acquires a tolerance for alcohol. That is, the amount which may be imbibed and the amount required to produce the characteristic effects first experienced gradually increase until very great quantities are sometimes required to satisfy the craving which its habitual use often produces. This is never the case with true foods.... Alcohol behaves in this regard just as does opium or any other drug. It has no resemblance to a food.
"3. When alcohol is withdrawn from a person who has been accustomed to its daily use, most distressing effects are experienced.... Who ever saw a man's hand trembling or his nervous system unstrung because he could not get a potato or a piece of cornbread for breakfast? In this respect, also, alcohol behaves like opium, cocaine, or any other enslaving drug.
"4. Alcohol lessens the appreciation and the value of brain and nerve activity, while food reënforces nervous and mental energy.
"5. Alcohol as a protoplasmic poison lessens muscular power, whereas food increases energy and endurance.
"6. Alcohol lessens the power to endure cold. This is true to such a marked degree that its use by persons accompanying Arctic expeditions is absolutely prohibited. Food, on the other hand, increases ability to endure cold. The temperature after taking food is raised. After taking alcohol, the temperature, as shown by the thermometer, is lowered.
"7. Alcohol cannot be stored in the body for future use, whereas all food substances can be so stored.
"8. Food burns slowly in the body, as it is required to satisfy the body's needs. Alcohol is readily oxidized and eliminated, the same as any other oxidizable drug."
Experiment (by Davison) to show how the nicotine in six cigarettes was sufficient to kill this fish. The smoke from the cigarettes was passed through the water in which the fish is swimming.
The Use of Tobacco.—A well-known authority defines a narcotic as a substance "which directly induces sleep, blunts the senses, and, in large amounts, produces complete insensibility." Tobacco, opium, chloral, and cocaine are examples of narcotics. Tobacco owes its narcotic influence to a strong poison known as nicotine. Its use in killing insect parasites on plants is well known. In experiments with jellyfish and other lowly organized animals, the author has found as small a per cent as one part of nicotine to one hundred thousand parts of sea water to be sufficient to profoundly affect an animal placed within it. The illustration here given shows the effect of nicotine upon a fish, one of the vertebrate animals. Nicotine in a pure form is so powerful a poison that two or three drops would be sufficient to cause the death of a man by its action upon the nervous system, especially the nerves controlling the beating of the heart. This action is well known among boys training for athletic contests. The heart is affected; boys become "short-winded" as a result of the action on the heart. It has been demonstrated that tobacco has, too, an important effect on muscular development. The stunted appearance of the young smoker is well known.
The amounts of alcohol in some liquors and in some patent medicines. a, beer, 5 %; b, claret, 8 %; c, champagne, 9 %; d, whisky, 50 %; e, well-known sarsaparilla, 18 %; f, g, h, much-advertised nerve tonics, 20 %, 21 %, 25 %; i, another much-advertised sarsaparilla, 27 %; j, a well-known tonic, 28 %; k, l, bitters, 37 %, 44 % alcohol.
Use and Abuse of Drugs.—The American people are addicted to the use of drugs, and especially patent medicines. A glance at the street-car advertisements shows this. Most of the medicines advertised contain alcohol in greater quantity than beer or wine, and many of them have opium, morphine, or cocaine in their composition. Paregoric and laudanum, medicines sometimes given to young children, are examples of dangerous drugs that contain opium. Dr. George D. Haggard of Minneapolis has shown by many analyses that a large number of the so-called "malts," "malt extracts," and "tonics," including several of the best known and most advertised on the market, are simply disguised beers and, frequently, very poor beers at that. These drugs, in addition to being harmful, affect the person using them in such a manner that he soon feels the need for the drug. Thus the drug habit is formed,—a condition which has wrecked thousands of lives. A number of articles on patent medicines recently appeared in a leading magazine and have been collected and published under the title of The Great American Fraud. In this booklet the author points out a number of different kinds of "cures" and patent medicines. The most dangerous are those headache or neuralgia cures containing acetanilid. This drug is a heart depresser and should not be used without medical advice. Another drug which is responsible for habit formation is cocaine. This is often found in catarrh or other cures. Alcohol is the basis of all tonics or "bracers." Every boy and girl should read this booklet so as to be forearmed against evils of the sort just described.
[36] Adapted from Atwater, Principles of Nutrition and Nutritive Value of Food, U. S. Department of Agriculture, 1902.
[37] and [38] W. O. Atwater, Principles of Nutrition and Nutritive Value of Food, U. S. Department of Agriculture, 1902.
[39] The above tables have been taken from the excellent pamphlet of the Cornell Reading Course, No. 6, Human Nutrition.
[40] Alcohol is made up of carbon, oxygen, and hydrogen. It is very easily oxidized, but it cannot, as is shown by the chemical formula, be of use to the body in tissue building, because of its lack of nitrogen.
[41] Alcoholic beverages contain the following proportions of alcohol: beer, from 2 to 5 per cent; wine, from 10 to 20 per cent; liquors, from 30 to 70 per cent. Patent medicines frequently contain as high as 60 per cent alcohol. (See page 294.)
Reference Reading on Foods
Hunter, Laboratory Problems in Civic Biology. American Book Company.
Allen, Civics and Health. Ginn and Company.
Bulletin 13, American School of Home Economics, Chicago.
Cornell University Reading Course, Buls. 6 and 7, Human Nutrition.
Davison, The Human Body and Health. American Book Company.
Jordan, The Principles of Human Nutrition. The Macmillan Company.
Kehler, L. F., Habit-forming Agents. Farmers' Bulletin 393, U. S. Dept. of Agri.
Lusk, Science and Nutrition. W. B. Saunders Company.
Norton, Foods and Dietetics. American School of Home Economics.
Olsen, Pure Foods. Ginn and Company.
Sharpe, A Laboratory Manual for the Solution of Problems in Biology, pp. 226-240. American Book Company.
Stiles, Nutritional Physiology. W. B. Saunders Company.
The Great American Fraud. American Medical Association, Chicago.
The Propaganda for Reform in Proprietary Medicines. Am. Medical Association.
Farmers' Bulletin: numbers 23, 34, 42, 85, 93, 121, 128, 132, 142, 182, 249, 295, 298.
Reprint from Yearbook, 1901, Atwater, Dietaries in Public Institutions.
Reprint from Yearbook, 1902, Milner, Cost of Food related to its Nutritive Value.
Experiment Station, Circular 46, Langworthy, Functions and Uses of Food.
XX. DIGESTION AND ABSORPTION
Problems.—To determine where digestion takes place by examining:—
(a) The functions of glands.
(b) The work done in the mouth.
(c) The work done in the stomach.
(d) The work done in the small intestine.
(e) The function of the liver.
To discover the absorbing apparatus and how it is used.
Laboratory Suggestions
Demonstration of food tube of man (manikin).—Comparison with food tube of frog. Drawing (comparative) of food tube and digestive glands of frog and man.
Demonstration of simple gland.—(Microscopic preparation.)
Home experiment and laboratory demonstration.—The digestion of starch by saliva. Conditions favorable and unfavorable.
Demonstration experiment.—The digestion of proteins with artificial gastric juice. Conditions favorable and unfavorable.
Demonstration.—An emulsion as seen under the compound microscope.
Demonstration.—Emulsification of fats with artificial pancreatic fluid. Digestion of starch and protein with artificial pancreatic fluid.
Demonstration of "tripe" to show increase of surface of digestive tube.
Laboratory or home exercise.—Make a table showing the changes produced upon food substances by each digestive fluid, the reaction (acid or alkaline) of the fluid, when the fluid acts, and what results from its action.
Purpose of Digestion.—We have learned that starch and protein food of plants are formed in the leaves. A plant, however, is unable to make use of the food in this condition. Before it can be transported from one part of the plant body to another, it is changed into a soluble form. In this state it can be passed from cell to cell by the process of osmosis. Much the same condition exists in animals. In order that food may be of use to man, it must be changed into a state that will allow of its passage in a soluble form through the walls of the alimentary canal, or food tube. This is done by the enzymes which cause digestion. It will be the purpose of this chapter to discover where and how digestion takes place in our own body.
The digestive tract of the frog and man. Gul, gullet; S, stomach; L, liver; G, gall bladder; P, pancreas; Sp, spleen; SI, small intestine; LI, large intestine; V, appendix; A, anus.
Alimentary Canal.—In all vertebrate animals, including man, food is taken in the mouth and passed through a food tube in which it is digested. This tube is composed of different portions, named, respectively, as we pass from the mouth downward, the gullet, stomach, small and large intestine, and rectum.
Comparison of Food Tube of a Frog and Man.—If we compare the food tube of a dissected frog with the food tube of man (as shown by a manikin or chart), we find part for part they are much the same. But we notice that the intestines of man, both small and large, are relatively longer than in the frog. We also notice in man the body cavity or space in which the internal organs rest is divided in two parts by a wall of muscle, the diaphragm, which separates the heart and lungs from the other internal organs. In the frog no muscular diaphragm exists. In the frog we can see plainly the silvery transparent mesentery or double fold of the lining of the body cavity in which the organs of digestion are suspended. Numerous blood vessels can be found especially in the walls of the food tube.
Glands.—In addition to the alimentary canal proper, we find a number of digestive glands, varying in size and position, connected with the canal.
Diagram of a gland. i, the common tube which carries off the secretions formed in the cells lining the cavity c; a, arteries carrying blood to the glands; v, veins taking blood away from the glands.
What a Gland Does. Enzymes.—In man there are the saliva gland of the mouth, the gastric glands of the stomach, the pancreas and liver, the two latter connected with the small intestine, and the intestinal glands in the walls of the intestine. Besides glands which aid in digestion there are several others of which we will speak later. As we have already learned, a gland is a collection of cells which takes up material from within the body and manufactures from it something which is later poured out as a secretion. An example of a gland in plants is found in the nectar-secreting cells of a flower.
Certain substances, called enzymes, formed by glands cause the digestion of food. The enzymes secreted by the cells of the glands and poured out into the food tube act upon insoluble foods so as to change them to a soluble form. They are the product of the activity of the cell, although they are not themselves alive. We do not know much about enzymes themselves, but we can observe what they do. Some enzymes render soluble different foods, others work in the blood, still others probably act within any cell of the body as an aid to oxidation, when work is done. Enzymes are very sensitive to changes in temperature and to the degree of acidity or alkalinity[42] of the material in which they act. We will find that the enzymes found in glands in the mouth will not act long in the stomach because of the change from an alkaline surrounding in the mouth to that of an acid in the stomach. Enzymes seem to be able to work indefinitely, providing the surroundings are favorable. A small amount of digestive fluid, if it had long enough to work, could therefore digest an indefinite amount of food.
Gland Structure.—The entire inner surface of the food tube is covered with a soft lining of mucous membrane. This is always moist because certain cells, called mucus cells, empty out their contents into the food tube, thus lubricating its inner surface. When a large number of cells which have the power to secrete fluids are collected together, the surface of the food tube may become indented at this point to form a pitlike gland. Often such depressions are branched, thus giving a greater secreting surface, as is seen in the figure on page 298. The cells of the gland are always supplied with blood vessels and nerves, for the secretions of the glands are under the control of the nervous system.
How a Gland Secretes.—We must therefore imagine that as the blood goes to the cells of a gland it there loses some substances which the gland cells take out and make over into the particular enzyme that they are called upon to manufacture. Under certain conditions, such as the sight or smell of food, or even the desire for it, the activity of the gland is stimulated. It then pours out its secretion containing the digestive enzyme. Thus a gland does its work.
Salivary Glands.—We are all familiar with the substance called saliva which acts as a lubricant in the mouth. Saliva is manufactured in the cells of three pairs of glands which empty into the mouth, and which are called, according to their position, the parotid (beside the ear), the submaxillary (under the jawbone), and the sublingual (under the tongue).
Digestion of Starch.—If we collect some saliva in a test tube, add to it a little starch paste, place the tube containing the mixture for a few minutes in tepid water, and then test with Fehling's solution, we shall find grape sugar present. Careful tests of the starch paste and of the saliva made separately will usually show no grape sugar in either.